Mapping the changing world of physics 1985-2009

Science is a social enterprise and as such is subject to fragmentation into groups or communities with strong interaction among members belonging to the same group and relatively weak interactions between those in different groups. Identifying communities corresponding to various scientific sub-fields or specializations through the analysis of formal and informal communication networks have been mainly pursued by analyzing the linkages revealed by citations among scientific papers (Solla Price, Redner). However, scientists may belong to multiple communities, either simultaneously or in succession, and a complementary method to identify multi- or inter-disciplinary communications between different fields of science is to focus on the self-categorization of their work by individual scientists. One such approach involves using the PACS codes used by papers published in the journals of American Physics Society (now adopted by others) which has been in use from the mid 1980s. As a paper can use several such codes to identify the sub-fields it relates to, this provides us with an opportunity to study the extent of inter-disciplinary interactions between apparently unrelated sub-fields in physics - and especially to see whether it has changed in nature and intensity over the past two decades that PACS usage has been wide-spread. While everybody suspects that cross-disciplinarity has increased over time (as mentioned often in faculty meetings, seminars and conferences) our work is an attempt to quantify this trend by studying the evolution of the network of PACS codes over time. Such a study of inter-linkages between different sub-fields can also potentially alert us to the development of new specializations.